Base stations today are known to employ energy-efficiency mechanisms that increase the energy efficiency of the entire network. One of these energy efficiency mechanisms is referred to as “cell-breathing”. When utilizing cell-breathing, a base station controls the coverage area of its associated cell to expand or shrink the coverage area as required due to the traffic load in the cell and in neighbor cell(s). When the traffic load in the cell begins to exceed the capacity of the base station, the base station may reduce its transmitter power to force User Equipments (UEs) operating near the cell border to handoff to neighboring cells, thus reducing the load on the base station. This effectively reduces the size of the cell. Conversely, when the traffic load decreases, the base station may increase its transmitter power, thus enabling additional UEs outside the cell border to operate within the cell and effectively increasing the size of the cell. An example of a cell-breathing algorithm is disclosed in the paper by M. Marsan et al, “Optimal Energy Savings in Cellular Access Networks”, Proceedings of ICC Workshop 2009, Dresden, June 2009.
In addition to energy management, base stations must also deal with mobility management for the UEs operating within the network. A UE in idle mode may camp or attach to a given cell. If the user, or equivalently the UE, moves to another cell, a cell reselection procedure must be performed. When the UE is active and moves to another cell, a handover procedure must be performed.
Existing mobility management procedures do not take into consideration the energy-efficiency mechanisms that many base stations utilize to maximize their energy efficiency. Mobility management without this input may result in either non-optimal mobility management or non-optimal energy efficiency. For example, a given base station might be switching off its transmitters and receivers for energy reduction reasons when a UE in idle mode tries to attach to the cell controlled by this base station. Alternatively, the UE might be attached to this designated cell when the base station reduces power, and this reduction leaves the UE temporarily or permanently out of coverage. If the UE cannot find another appropriate cell in the vicinity, it may be dropped from the network entirely. Even if the UE is not dropped, the UE has to search for another cell to camp on. This results in higher energy consumption at the UE. It is readily understood, that this results in reduced or degraded service for users in the network.
In another scenario, a given UE may be ready to perform handover from a serving base station to a target base station, which is about to reduce or shut down its transmitters/receivers for the sake of minimizing energy consumption in the network. Within protocols such as 3GPP LTE (Advanced), the serving base station sends a Handover Request message to the target base station via the X2 interface (as described in TS 36.423), requesting whether the target base station can accept the UE. If the target base station is performing a procedure to switch off its transmitters/receivers, the target base station may refuse to accept the UE. This means that a number of pointless signaling messages are exchanged via the X2 interface, resulting in an unnecessary load on this interface.